The transmission of ground motion, such as caused by earthquakes, to structures has been attempted to be controlled through isolation of the structure at its base. This has involved specially designed foundation systems that limit the intensity of the ground motion transmitted to the superstructures. As noted in the prior art, prior traditional methods of providing for lateral load-carrying capacity allow the entire ground motion from an earthquake to be transmitted to the superstructure and then attempt to provide for the absorption of the energy through inelastic structural materials, which inevitably gives rise to damage to structural and nonstructural elements.
The concept of limiting the intensity of motions transmitted to the structures via certain base isolation schemes has been explored in the prior art. Besides the classical spring-mass system, whic is based on harmonic excitations, there are many other suggestions in the art to isolate structures from the damaging effects of earthquakes. The flexible first story concept and the soft story concept, due to consequential instability and P .DELTA. effects, are not practical schemes. The use of ball bearings and specially shaped rollers under the structures has also been suggested.
Considerable work has been done to show the effectiveness of steel plate laminated rubber bearings with and without a lead core as a base isolator system. To limit the shear distortion in the elastomer and to bestow larger displacement capability, the use of a friction plate in conjunction with steel plate laminated rubber bearings has also been considered. If the predominant frequency of excitation is low (as in the case of a soft site), the above systems can act as amplifiers and impose larger displacement demands on the base.
After the ground motions of a typical earthquake stop, there are residual displacements in the sliding type isolators. Since these residual displacements can increase in subsequent earthquakes, repositioning operations become necessary. It would therefore be desirable to provide a base isolator system which is endowed with elastic elements which limit the maximum residual displacement to a constant specified quantity and eleminate or reduce the need for repositioning operations.